Structure and dynamics of ammonium borohydride.

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• Ammonium borohydride (NH4BH4) is a material with potential applications in hydrogen storage.
• Understanding its crystalline structure, particularly the hydrogen atom positions, is crucial for optimizing its properties.
• Previous structural analyses may not have fully captured the dynamic behavior of the ions.

Purpose of the Study:

• To precisely determine the crystal structure of NH4BH4, including the location of hydrogen atoms.
• To investigate the dynamic behavior of ammonium (NH4+) and borohydride (BH4-) ions in the crystalline state.
• To provide a refined structural model for NH4BH4.

Main Methods:

• Synchrotron powder X-ray diffraction was employed for high-resolution structural analysis.
• Ab initio molecular dynamics calculations were performed to simulate ion behavior.
• Solid-state deuterium (2H) and proton (1H) Nuclear Magnetic Resonance (NMR) spectroscopy was used to probe local environments and dynamics.

Main Results:

• The crystal structure of NH4BH4 was refined, accurately locating the hydrogen atoms.
• Evidence of rapid reorientations of both NH4+ and BH4- ions was observed.
• On average, hydrogen atoms were found to occupy half of the positions at the corners of a cube around the central B or N atoms.

Conclusions:

• The dynamic nature of hydrogen atoms in NH4BH4 is a key feature of its crystalline structure.
• The refined structural model provides a better understanding of NH4BH4's properties.
• This work contributes to the fundamental knowledge of hydrogen-rich materials for potential applications.